1. Field of the Invention
The present invention relates to an acoustic method and an apparatus for measuring the horizontal location and/or the depth of underground pipe and conduit. It is especially useful for nonmetallic or nonconductive pipe such as sewer, gas and water pipes.
2. Brief Description of the Prior Art
Most underground utility pipe and conduit are located within 10 ft of the surface of the earth. The majority of this piping is found in the upper three feet. The number, location and depth of these utility lines are often unknown. Engineering drawings are sometimes incomplete, out of date, or inaccurate. Because of the existence and increased use of nonmetallic pipe and conduit, and the fact that horizontal drilling is replacing traditional trenching methods, the need for pipe and conduit detectors is even more important today than it has been in the past. Pipe and conduit detectors can play an important role in utility planning, construction and bidding; they are useful tools for obstacle avoidance in both open trenching and horizontal drilling operations.
There are many methods available for detecting metallic pipe. These so-called "electromagnetic" methods, however, do not work on nonmetallic pipe. Yet, most of the utility pipe in use and now being installed (for example, sewer, gas and water) is made of nonmetallic materials. This is also true of many other types of piping (e.g., underground petroleum piping found at retail service stations). Detection of sewer pipe, especially laterals, is particularly a problem, because both the older and newer pipes are nonmetallic. Acoustic methods have recently been introduced as a means of detecting the horizontal location of nonmetallic pipe, but no acoustic method has been used to measure depth. In this specification, the projected position of the buried pipe on the surface of the ground will be referred to as "location," "horizontal location," "horizontal surface location," or "surface position." Location is a two-dimensional ground-surface measurement and does not include a measurement of depth. Location and depth of the pipe are referred to separately in this specification. Together they define the three-dimensional location of the underground pipe.
Most of the acoustic systems described in the prior art work as follows. First, an acoustic signal--in the form of a pulse, chirp or continuous wave (CW)--is injected into the pipe. As the signal travels along the length of the pipe, a portion of its acoustic energy propagates outward and into the surrounding soil. Because of the high attenuation of acoustic signals in soil at higher frequencies, the source frequencies are usually less than 3 kHz and more typically between 100 and 500 Hz. Second, receive sensors are mounted on the ground surface, or inserted a few inches deep, at a number of locations in the putative vicinity of the buried pipe until the acoustic signal propagating from the pipe is detected. The location of the pipe is determined from the magnitude of the signal. The location is associated with the strongest received signal. Accelerometers and geophones have been used successfully as receive sensors in the field.
Most commercially available acoustic systems are based on magnitude measurements. U.S. Pat. Nos. 5,491,012, 5,036,497, 5,452,263, and 5,412,989 teach methods for locating underground pipe based on the magnitude (or intensity) of the received acoustic signal. In general, these systems differ mainly in the method of acoustically exciting the pipe.
In U.S. Pat. No. 5,491,012, Ziska presents a method for excavating an underground sewer pipe that includes an acoustic technique for locating that pipe. A source of sound is introduced into the sewer line, and a detection sensor is moved along the surface of the ground; the strongest signal detected indicates the location of the pipe.
In U.S. Pat. Nos. 5,036,497 and 5,452,263, Heitman presents an acoustic method for locating water pipes and other types of pressurized lines. In this invention, the pipe is excited by a pressure transient wave (shock wave), which is produced by quickly opening and closing a valve on the pipe. At the surface, one or two sensors measure the magnitude of the signal in order to locate the pipe. (If one sensor is used, it is the peak signal that indicates the location of the pipe; if two sensors are used, the location is indicated when the magnitude of the received signal is the same at both sensors.)
In U.S. Pat. No. 5,412,989, Eberle et al. describe an acoustic system for locating buried gas pipe and other nonmetallic pipe in the upper meter of the ground that is also based on the magnitude of the received signal. In this invention, the sound source is a broad band of frequencies (e.g., a swept sine excitation from 100 to 1,000 Hz). Eberle claims that this means of excitation produces a signal at the surface of the ground that has a higher signal-to-noise ratio (SNR) than excitation using only a single frequency (e.g., 400 Hz). The location estimates are based on the mean square signal strength.
Location systems that measure the magnitude of the signal can be subject to large errors. The peak magnitude is dependent not only on the strength of the signal but also on sensor-to-ground coupling. If there is inconsistent coupling, it is possible that the signal received by a sensor located directly above a buried pipe may actually be weaker than one received by a sensor farther away. Interpretation of the received signal can be further confused, because man-made and topographical anomalies will also affect the magnitude of the signal. Thus in practice it is possible that no clearly defined peak or signal maximum will be found, and equal signals may not occur at equal offsets from the pipe. This can lead to large errors in the horizontal location of the pipe. In some instances, the magnitude of the received signal is so ambiguous that false and missed detections occur. The applicability of location systems that measure the magnitude of the signal tends to be limited to very shallow depths (that is, to within 3 ft of the ground surface).
In U.S. Pat. No. 5,127,267, Huebler avoids the problems of sensor-to-ground coupling and other factors affecting the strength of the acoustic signal strength. He uses the time of arrival of an identifiable acoustic signal to locate the pipe. For a signal to be identifiable, his invention requires that the time between successive signals be long enough that one signal can be received before the next is transmitted. For this method to work, the time of the transmitted signal relative to that of the received signal must be known. The location of the pipe is assumed to be directly beneath the sensor that receives the signal in the shortest time (that is, the "minimum time of arrival"). If an array of sensors is used, the same transmitted signal must be received at multiple locations so that arrival times can be compared. The main advantage of Huebler's approach is that it minimizes sensor coupling issues. It is not obvious or necessary, however, that the shortest travel time corresponds to the closest geometrical position of the pipe. The travel time of the acoustic signal propagating through the pipe may take a quicker path through the ground and be detected at a position off the center of the pipe. This will occur, for example, if the propagation velocity of the signal through the pipe is less than through the surrounding soil. Actually, there is no unique place on the surface for the maximum signal to occur. Accurate location of the pipe can only be made analyzing acoustic data from both sides of the pipe.
The method and apparatus of the present invention addresses the problems encountered with both the magnitude-based and time-of-arrival-based systems described above and has a number of important advantages over them. First, it addresses the location accuracy problems of both measurement approaches. Second, it avoids the sensor coupling issues inherent in the magnitude-based systems. Third, the present invention provides a quantity not measured by any other acoustic system--an estimate of the depth of the pipe.